Re: [TCP_CA_CWR] Causes for entering TCP_CA_CWR state with 0 retransmissions

[Yinglin Sun <yinglinsun@cs.pitt.edu>]

Hi David,

I use the total number of retransmissions reported by tcp_info. The
field name is tcpi_total_retrans. From the kernel source code, I found
that this number is for both fast retransmit and timeout
retransmissions. On the other hand, if fast retransmission happened,
ca_state should be TCP_CA_Recovery or TCP_CA_Disorder, but it's
TCP_CA_CWR.

Thanks.

Yinglin

On Tue, Jul 14, 2009 at 2:42 PM, David Miller<davem@davemloft.net> wrote:
> From: Yinglin Sun <yinglin.s@gmail.com>
> Date: Tue, 14 Jul 2009 14:38:07 -0400
>
>> I have a question about tcp_ca_state "TCP_CA_CWR".  Under what
>> circumstances does it enter this state?
>> I'm doing some experiments and have trace from tcp_info. From tcp_info
>> trace, I found that many places show ca_state is 2 (TCP_CA_CWR). Under
>> this state, the congestion window size decreases but the total number
>> of retransmissions is still 0.
>
> What kind of "retransmissions"?  I bet the counter you are looking
> at is counting timeout based retransmissions, which means it isn't
> including the most predominant type of retransmissions, which are
> fast retransmissions which are triggered in response to ACKs.
>